Field of the Invention
The present Invention relates to an external resonator type frequency-variable semiconductor laser light source for optical coherent communication, which varies the oscillation frequency of the semiconductor laser while continuing the phase of oscillated light.
Background Art
FIG. 2 is a block diagram showing a conventional external resonator type frequency-variable semiconductor laser light source. In FIG. 2, reference numeral 1 indicates a semiconductor laser, reference numeral 1A indicates an antireflection film reference numeral 3 indicates a diffraction grating reference numeral 4 indicates a rotating stage, reference numeral 5 indicates a parallel sliding stage, reference numerals 6 and 7 indicate lenses, reference numeral 9 indicates a frequency setting part, reference numeral 10 indicates a comparator, reference numeral 11 indicates a parallel sliding mechanism driver, reference numeral 12 indicates a fixed plate, reference numeral 13 indicates an arm, and reference numeral 17 indicates a displacement gauge.
In the arrangement of FIG. 2, one end face of semiconductor laser 1 is coated with antireflection film 1A. From the end face with tile antireflection film, outgoing beam 2B is outputted. The outgoing beam 2B is transformed into a collimated beam via lens 6 and is incident on diffraction grating 3. At this time, diffraction grating 3 and the other end face without an antireflection film of the semiconductor laser form an external resonator whose length is "L". Semiconductor laser 1 oscillates at a single mode and outputs outgoing beam 2A from the other end face.
Here, diffraction grating 3 is fixed on rotating stage 4 and the rotating stage 4 is further fixed on parallel sliding stage 5 which moves in parallel with the light axis of semiconductor laser 1. Furthermore, the rotating stage 4 is in contact with fixed plate 12 via arm 13. Therefore, the parallel motion of parallel sliding stage 5 is transformed to the rotational motion of rotating stage 4; thus, the oscillation frequency of semiconductor 1 is varied under phase-continuous condition by way of the parallel movement of the parallel sliding stage 5.
Frequency setting part 9 sets up the oscillation frequency of semiconductor 1. A set signal from frequency setting part 9 and a displacement signal from displacement gauge 17 which detects the amount of the parallel displacement of parallel sliding stage 5 are compared by comparator 10; and the comparison signal is fed as a control signal back to parallel sliding mechanism driver 11. In this way, the oscillation frequency of semiconductor laser 1 is set arbitrarily within the resolution of displacement gauge 17 under phase-continuous conditions.
On the other hand, outgoing beam 2A from the other end face without an antireflection film of semiconductor laser 1 is transformed into a collimated beam; and the collimated beam becomes an output beam from the external resonator type frequency-variable semiconductor laser light source.
The set resolution of the oscillation frequency of the semiconductor laser is limited by the resolution of the rotation angle of the diffraction grating which acts as an external mirror of the semiconductor laser; thus, in order to raise the set resolution of the oscillation frequency, it is necessary to raise the resolution of the rotation angle of the diffraction grating.
However, in the arrangement of FIG. 2, the rotation angle .theta. of the diffraction grating is calculated in accordance with the amount of the parallel displacement of the parallel sliding stage; thus, tile set resolution of the oscillation frequency of the semiconductor laser is limited by the resolution of the displacement gauge which detects the amount of the parallel displacement of the parallel sliding stage.
At present, the resolution of the displacement gauge regarded as one having the highest resolution is 20 nm at its best, even if using a strain gauge and so on. Therefore, a problem occurs in that the oscillation frequency of the semiconductor laser cannot be set up at a better resolution than one corresponding to the resolution (20 nm) of the displacement gauge.